The two expressions are not quite the same but there is no difference in politeness. They are equally polite. I agree with Marie-Helene. If you say: 'That's about all' you haven't quite finished what you are saying or doing but you've done most of it, especially the things that are important. e.g. At the end of the day at work. I might say. 'That's all. I'm going home now. I would then do no more work and would go home. Another day, I might day, 'That's about all. I just need to send a few emails and then I'm going home'

Hello Irina, "That's about all" means "that's near enough to all" "That's all" = that's it Hope you can hear the difference...

They are meaning almost indentical. The two words are all express meanting .so you said that sentence.